Switching devices, in particular low-voltage switching devices, can be used to switch the current paths between an electrical supply device and loads, and therefore their operating currents. Thus, the switching device opens and closes current paths such that connected loads can be connected and disconnected safely.
An electrical low-voltage switching device such as a contactor, a circuit breaker or a compact starter has one or more so-called main contacts, which can be controlled by one or more control magnets, for switching the current paths. In principle, the main contacts in this case comprise a moving contact link and fixed contact pieces, to which the load and the supply device are connected. An appropriate connection or disconnection signal is passed to the control magnets in order to close and open the main contacts, in response to which the armatures of these control magnets act on the moving contact links such that the contact links carry out a relative movement with respect to the fixed contact piece and neither close nor open the current paths to be switched.
In order to make better contact between the contact pieces and the contact links, appropriately designed contact surfaces are provided at points at which the two meet one another. These contact surfaces are composed of materials such as silver alloys which at these points are applied both to the contact link and to the contact pieces, and have a specific thickness.
The materials of the contact surfaces are subject to wear during every switching process. Factors which an influence this wear are:                Increasing contact erosion or contact wear as the number of connection and disconnection processes increases,        Increasing deformation,        Increasing contact corrosion caused by arcing, or        Environmental influences, such as vapors or suspended particles, etc.        
Thus, the operating currents are no longer safely switched, and this can lead to current interruptions, contact heating or to contact welding.
For example, particularly as the contact erosion increases, the thickness of the materials applied to the contact surfaces is decreased. In consequence, the switching movement between the contact surfaces of the contact link and the contact pieces becomes longer, thus in the end reducing the contact force on closing. As a consequence of this, the contacts no longer close correctly as the number of switching processes increases. The current interruptions resulting from this or else increased connection bouncing can lead to contact heating and thus to increasing melting of the contact material, which can in turn lead to the contact surfaces of the main contacts being welded.
If a main contact in the switching device is worn or welded, the switching device can no longer safely disconnect the load. For example, particularly in the case of a welded contact, at least the current path with the welded main contact will continue to carry currents and be live despite the disconnection signal, so that the load is not completely disconnected from the supply device. Since the load therefore remains in a non-safe state, the switching device represents a potential fault source.
In the case of compact starters according to IEC 60 947-6-2, for example, in which an additional protective mechanism acts on the same main contacts as the control magnet during normal switching, for example, the protective function can thus be blocked.
Fault sources such as these must therefore be avoided for safe operation of switching devices and therefore for protection of the load and of the electrical installation.